Take a look at this new work by Richard Leeke for your circles -- Richard Leeke's New Version of (Concentric) Circles Unfortunately (perhaps) you will need software like MapInfo to be able to capture (by centroid) or retrieve and aggregate the population numbers across the SA1's under each circle. I'm less familiar with the open source GIS packages out there that may have that functionality but perhaps someone on this forum can suggest one. Of course, optimizing your service centre network is an even more challenging spatial analysis problem. Maybe Mr. Leeke and his colleagues over in NZ can provide that service for a fee. If not, call the Pitney Bowes office in Melbourne for some direction.
Best of luck,